def file also says it is a repeatable trigger. trigger_multiple & once both are the same spawnclass & the def for trigger_once defaults a wait of "-1" which means only trigger once. So it's just a def that defaults to triggering once but can be used multiple times.

You can set any spawnargs you want to the entity & only the recognizable ones will be used by the game. So, if you have a trigger with the spawnarg "stupid 1" & used "getkey" on "stupid" it should result in 1.

I haven't tested this in a while but I want to say I've done it before to store values in entities.

Well, normally triggers activate something, use a script that every X seconds checks for a condition & when the condition is met, activate whatever the trigger would. If you want multiple things activated at once have the script call a trigger_relay.